1. Technical Field of the Invention
The present invention relates to the field of DC/DC converters. In one aspect, the present invention relates to a method and apparatus for converting a first voltage to a second predetermined voltage under the control of synchronous switching to provide continuous AC current, low noise DC voltage conversion.
2. Description of Related Art
DC/DC converters by convention are classified in one of five types; buck, boost, buck-boost, SEPIC (single-ended parallel inductor converter), and Cuk. These converters are composed of at least one switch, diode, inductor, and capacitor in various configurations to convert an input DC voltage into an output DC voltage. In their simplest form they are non-isolated but with the addition of a transformer isolation can be provided without altering basic properties. Each type converter has unique characteristics that give it an advantage in a particular application.
FIG. 1 illustrates a non-isolated buck converter having a MOSFET power transistor Q1 switching on and off with a percentage on-time duty cycle d, an inductor L1, diode D1, and capacitor C1. The output voltage relates to the input voltage by the equation Vo=Vi.times.d. This type converter is used when the input voltage must be stepped-down. The input voltage must always be greater than the output voltage for proper operation. The inductor in series with the output node provides smoothing of the AC ripple current reducing output noise. However, the input is characterized by high AC ripple current.
FIG. 2 illustrates a non-isolated boost converter having transistor Q2, inductor L2, diode D2, and capacitor C2. The output voltage relates to the input voltage by the equation Vo=Vi/(1-d). This type converter is used when the input voltage must be stepped-up. The input voltage must always be less than output voltage for proper operation. The inductor in series with the input node provides smoothing of the AC ripple current reducing input noise. However, the output is characterized by high AC ripple current.
FIG. 3 illustrates a non-isolated buck-boost converter having transistor Q3, inductor L3, diode D3, and capacitor C3. The output voltage relates to the input voltage by the equation Vo=-Vi.times.d/(1-d). This type converter is operable with input voltages that are greater than, less than, or equal to the output voltage. However, the output polarity is inverted. Changing the inductor into a transformer could correct the inversion making it a conventional flyback converter. Also, there is no inductor in series with the input or output nodes to provide smoothing. Both input and output are characterized by high AC ripple current.
FIG. 4 illustrates a non-isolated SEPIC converter having a transistor Q4, inductors L4 and L5, diode D4, and capacitors C4 and C5. The output voltage is related to the input voltage by the equation Vo=Vi.times.d/(1-d). This type converter is operable with input voltages that are greater than, less than, or equal to the output voltage. The inductor in series with the input node provides smoothing of the AC ripple current reducing input noise. However, the output is characterized by high AC ripple current.
FIG. 5 illustrates a non-isolated Cuk converter having a transistor Q5, inductors L6 and L7, diode D5, and capacitors C6 and C7. The output voltage is related to the input voltage by the equation Vo=-Vi.times.d/(1-d). This type converter is operable with input voltages that are greater than, less than, or equal to the output voltage. However, the output polarity is inverted. An additional transformer is required to correct the inversion. The inductors in series with both input and output nodes provide smoothing of the AC ripple current reducing both input and output noise.
FIG. 5a illustrates a transformer-isolated two-transistor forward converter having transistors Q6 and Q7, diodes D6, D7, D8, and D9, transformer T1, inductor L8, and capacitor C8. The transistors are on and off simultaneously. When the transistors are on the input voltage is applied across the primary winding and transformed across the secondary winding proportionately as the ratio of primary to secondary turns on the transformer, n. The secondary voltage charges inductor L8 through diode D8 and capacitor C8. When the transistors are off the magnetizing energy is discharged through D6 and D7 clamping the primary winding to the input voltage in the opposite polarity. During the off period inductor L8 discharges through diode D9 and capacitor C8. The output voltage across C8 is related to the input voltage, transformer turns ratio n, and duty cycle d by the equation Vo=Vi.times.d/n. In order to maintain balanced magnetic flux in the transformer the maximum duty cycle d is limited to 50%. This type of converter is applicable when electrical isolation is required or when the input voltage is much greater than or much less than the output voltage. The inductor in series with the output provides smoothing of the AC ripple current reducing output noise. However, the input is characterized by high AC ripple current.
FIG. 5b illustrates a transformer-isolated single transistor forward converter having transistor Q8, diodes D10, D11, and D12, transformer T2 with a primary winding and two secondary windings, inductor L9, and capacitor C9. When the transistor is on the input voltage is applied across the primary winding and transformed across the first secondary winding proportionately as the ratio of primary to secondary turns on the transformer, n. The secondary circuitry operates the same as in the two-transistor forward converter. When the transistor is off the magnetizing energy is discharged through a second secondary winding and D10 clamping the winding to the input voltage in the opposite polarity. The second secondary winding typically has the same number of turns as the primary winding. In order to maintain balanced magnetic flux in the transformer the maximum duty cycle d is limited to 50%. This type converter has typically the same performance characteristics as the two-transistor forward converter. It has fewer components but the transistor has greater voltage stress than the two-ransistor version and the transformer requires an additional winding.
The demands of battery-sourced, battery-backed, and distributed power systems with point-of-load regulation have placed greater requirements for wide-range input (step-up/step-down) DC/DC converters with low noise inputs and outputs in small economical packages. The conventional converters illustrated above fall short in at least one of those respects. The buck and boost converters are limited by the range of input voltages which restricts the choice of battery cell count or system buss voltage. Input and output noise in any converter can be improved with additional filtering but it adds to the overall size and cost of the DC/DC converter. The Cuk converter is a wide-range input (step-up/step-down) with low noise inputs and outputs but has an inverted output.
Further limitations and disadvantages of conventional systems will become apparent to one of skill in the art after reviewing the remainder of the present application with reference to the drawings and detailed description which follow.